Distortionmeter
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Distortionmeter (or more precisely distortion factor meter) is an electronic measuring instrument which displays the amount of distortion added to the original signal by an electronic circuit.


Harmonic distortion

Harmonic distortion In signal processing, distortion is the alteration of the original shape (or other characteristic) of a signal. In communications and electronics it means the alteration of the waveform of an information-bearing signal, such as an audio signal ...
is equivalent to adding harmonics to a signal. When a purely sinusoidal signal is in this way, a series of harmonics is superimposed on the original signal, and can be detected with suitable equipment. If the input is : =a_1 \cdot sin(\omega t) The normalized output is : =a_1 \cdot sin(\omega t)+a_2 \cdot sin(2 \omega t)+a_3 \cdot sin(3 \omega t)+.. The value of Total Harmonics Distortion (THD) is defined as the ratio of the harmonics to the fundamental; Reference data for Radio Engineers, Howard W.Sams & co.ITT, , p.18-12 i.e., : THD = \frac This ratio can be given in dB or in percentage.


The instrument

A distortionmeter is actually a levelmeter with two switchable parallel circuits at the input. The first circuit measures the total signal at the output of a system. (For low distortion levels this will be almost equal to fundamental). That value is adjusted to read 100% or, equivalently, to 0 dB. The second circuit is a high pass filter which removes (as much as practical) the fundamental frequency. This can be a notch filter, one which passes all but the fundamental, with negligible attenuation at other frequencies (including whatever harmonics might be present). Alternatively, if the distortion products are at higher frequencies, a
highpass filter A high-pass filter (HPF) is an electronic filter that passes signals with a frequency higher than a certain cutoff frequency and attenuates signals with frequencies lower than the cutoff frequency. The amount of attenuation for each frequency d ...
can be used if its cutoff rate is sufficiently steep to not affect the expected distortion products. The output of the filter is measured as a percentage of the fundamental, and the reported value will be the distortion value.


See also

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Audio analyzer An audio analyzer is a test and measurement instrument used to objectively quantify the audio quality measurement, audio performance of electronic and electro-acoustical devices. Audio quality metrics cover a wide variety of parameters, including a ...


References

{{Electrical and electronic measuring equipment Electronic test equipment Broadcast engineering